On the other hand, it is
possible to extend the set of the quantum
fields, that is include other fields along with the metric, or even
replacing the metric. One of the most interesting approaches to
make such extension is to provide an extended symmetry, as it is
done in supergravity. We can refer the reader to the corresponding
Section of the present Handbook for the reviews of supergravity,
but will not discuss it here. Another possibility is to add quantum
matter fields. Let us note that there is a reduced, semiclassical
approach, when only the matter fields are assumed quantum and
metric is regarded as a classical background. This kind of theories
share many technical and conceptual features with perturbative QG,
so the reader may find useful to study the semiclassical approach
first. There are many useful monographs on this subject, let us
mention just a few of them,
[1]}, [2]}, [3]}, [4]}, [5]}, [6]} and also recent
textbook [7]}, where the reader can also find an introduction
to QGFor the interested reader, there are good books
and reviews of semiclassical and quantum gravity theories, e.g.,
[8]}, [9]}, [10]}, [11]}. One can find many
other useful sources starting form these references..
In what follows, we shall address the same subjects which
are discussed in sections 18-21 of this textbook. Most of the present
Chapter can be seen as a modified and more review-style extract
from this book.
